Abstract 1632: Preclinical characterization of DBXO-1, a multi-pMHC targeted bispecific T cell engager for major solid tumors

Abstract Bispecific T cell engagers (TCEs) are a well-established therapeutic modality in heme malignancies, and recent regulatory approvals in select solid tumors highlight their promise more broadly. However, adoption in major solid tumor indications and across large patient populations remains limited, in part by the scarcity of surface targets with cancer-exclusive expression patterns. Cancer-specific intracellular targets can be accessed by TCEs via peptide-MHC complexes (pMHCs), but individual pMHCs are expressed at low copy numbers which impacts TCE potency and breadth of response. DBXO-1 is designed to address this limitation by targeting a defined set of cancer-restricted intracellular proteins that generate multiple target pMHCs that can be recognized by a single binder, addressing copy number and heterogeneity concerns. Here, we describe the specificity profile and functional activity of DBXO-1 preclinically.A T cell receptor recognizing HLA-A*02:01-restricted target peptides was affinity- and specificity-engineered using yeast and phage display, aided by structural modelling. Optimized binders were formatted into Fc-containing TCEs using clinically validated anti-CD3 sequences. Affinity was measured by biolayer interferometry. Specificity was characterized using yeast display workflows and a T2/Jurkat reporter assay, cytotoxicity was assessed by LDH release. The DBXO-1 discovery campaign identified multiple pMHC binders with single digit nanomolar affinities to the relevant target pMHCs. When formatted into TCEs, this led to potent T cell activation with EC50s 10 pM. Mutational analyses confirmed a shared structural recognition mode across target pMHCs, underlining the basis for the intended multi-targeting mechanism. Importantly, the specificity of final leads was assessed sequence-agnostically by testing binding against 13,849 HLA-A*02:01-presented peptides that were found in healthy tissues by immunopeptidomics. This comprehensive assessment revealed only a very small number of low-affinity interactions, a profile superior to a clinical stage benchmark pMHC-TCE. When T2 cells were pulsed with these peptides at supraphysiological concentrations for additional stringency, 100x EC50 windows for T cell activation were observed. Further derisking on healthy cells is ongoing. Finally, DBXO-1 mediated potent, dose-dependent cytotoxicity on multiple HLA-A*02:01+ cell lines expressing the target signature, while no activity was observed in absence of either target expression or HLA restriction.Taken together, these data support continued preclinical development of DBXO-1, with the goal of entering a first-in-human clinical trial in 2027. Initial clinical development is anticipated in biomarker-selected patient populations across major solid tumors, including non-small cell lung, head and neck, and gastroesophageal cancers. Citation Format: Johanna K. Kaufmann, Jason Lajoie, John L. Silberstein, . Preclinical characterization of DBXO-1, a multi-pMHC targeted bispecific T cell engager for major solid tumors abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1632.